74HC21D Integrated Circuits (ICs)

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Engineering designs are often differentiated by their performance level, cost and size. The 74HC21D, a quad 2-input NAND gate, is among the small and inexpensive solutions available in the market and is distinguished by its fast operating speed and relatively low power consumption. It is widely used in various applications and some of those use cases are explained in this article. Furthermore, the article outlines working principle and pin-out configuration of the gate.

Overview of 74HC21D

The 74HC21D is a high-speed CMOS quad 2-input NAND gate, manufactured by Texas Instruments(TI). It operates over the voltage range of -0.5 to 7V, and achieve a switching speed of 4V/ns and a propagation delay of 2.2ns. The ±24mA output source/sink current is sufficient to drive TTL loads. It can also be used for Schottky or low noise implementations. Its low power consumption and low quiescent current make it suitable for low-power applications. This logic gate can be used with any logic or voltage level and it is CMOS compatible, meaning it can be intermixed with LSTTL logic devices with no modifications.

Configuration and Pin-out

The 74HC21D is composed of four independent 2-input NAND gates with Schmitt-trigger inputs, which means the inputs can be interfaced with slow-moving or noisy signals. It is available in SOP and SOIC packages, with standard pin-out configuration. As shown in the figure below, it features nine pins, with four inputs and four outputs.

Application Field of 74HC21D

Due to its wide operating range and low power consumption, the 74HC21D is widely used in various applications. Some of those applications are:

• Data Logging Systems: The 74HC21D can be used to convert slow-going analog data into digital information that can be processed and stored.
• Switch Debouncing: As it features Schmitt-trigger inputs, it is useful for eliminating contact bounce in switch input application.
• Counter Circuits: As a NAND gate, it is suitable for generating a clock pulse in counter circuits.
• Data Processing: The gate is suitable for reception and data processing of rapidly changing signals.

Working Principle of 74HC21D

The logic high or low in the output of the NAND gate is based on the logic value in the inputs. A logic high output can only be achieved when both inputs are logic lows. The output will remain logic high until both of the inputs become logic high, which triggers a logic low output. When the input transitions from a high voltage to low voltage, the output will rise to logic high in a very short amount of time. Similarly, when the input transitions from the low voltage to a higher voltage, the output will become low very quickly. This is the working principle of the NAND gate.

Conclusion

A 74HC21D gate is a quad 2-input NAND gate, which integrates four independent gates with Schmitt-trigger inputs. It has a wide operating voltage range, a low power consumption and a fast operating speed. It is best suited for debouncing of switch inputs, data logging and processing systems, as well as counter circuits. Its low propagation delay, meant for high performance, is also beneficial for low power designs.

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